1. Field of the Invention
The present invention, relates to a flow control device which includes a flow control valve in order to regulate the flow of a fluid. This present invention especially relates to a flow control device which has a linear variable differential transformer for detecting the lift of the flow control valve.
2. Description of the Prior Art
A flow control valve is installed to control the flow of a liquid. In order to detect the lift of the flow control valve, conventionally a position transmitter or the like is used. A position transmitter used for detection of the valve lift converts a vertical motion of a valve cylinder, which opens and closes the flow control valve, into a rotating motion and this rotating motion fluctuates the value of current flowing through a magnetic coil. By measuring the fluctuated current value, the position of the valve cylinder is detected.
However, in case of using the position transmitter, because the position is measured by converting the vertical motion of the valve cylinder into the rotating motion, the as-measured value is not a linear value in relation to the position of the valve cylinder. Therefore, precision in detecting the valve lift is inferior. Additionally, this rotating motion wears sliding portions such as the rotating shaft, bearings or the like, bringing about problems of a change of the rotation angle initially set at the time of installation and deterioration of the precision in detection.
In order to solve these problems, a xe2x80x9clinear variable differential transformerxe2x80x9d (LVDT) is used to detect the position of the valve cylinder by directly measuring the vertical motion of the valve cylinder. The internal configuration of a flow control device equipped with this LVDT is shown in FIG. 5. In FIG. 5, two LVDT""s 100a and 100b are installed for a valve cylinder 1a which is included in a flow control valve 1. Here, the LVDT 100a is provided with an iron core 104a interlocking with the valve cylinder 1a and a primary coil 101a, a secondary coil 102a and a secondary coil 103a that are wound around the iron core 104a. Also, the LVDT 100b is provided with an iron core 104b interlocking with the valve cylinder 1a, a primary coil 101b and secondary coils 102b and 103b wound around the iron core 104b. 
In the LVDT 100a, the secondary coils 102a and 103a are wound in inverse directions, and are series connected. The voltages generated at both ends of this series circuit of the secondary coils 102a and 103a are input into a servo controller 105a. At this time, an alternating current voltage is fed to the primary coil 101a by the servo controller 105a, generating induced voltages in the secondary coils 102a and 103a in accordance with the magnetic field generated by the primary coil 101a. 
Then, as the iron core 104a moves inside the secondary coils 102a and 103a, the inductance of each of the secondary coils 102a and 103a varies, depending on the position of the iron core 104a. Since the coils are wound in inverse directions, the voltages generated in the secondary coils 102a and 103a respectively cancel each other and their differential voltage is input into the servo controller 105a. Likewise, in the LVDT 100b, the same operation as in the LVDT 100a is performed, so that the same differential voltage is input into the servo controllers 105a and 105b by the LVDT""s 100a and 100b. 
With this configuration, the differential voltages generated by the LVDT""s 100a and 100b linearly express the position of the valve cylinder 1a, and precision in measuring the position of the valve cylinder 1a in accordance with the values detected from the differential voltages is improved. Additionally, in the servo controllers 105a and 105b, by comparing the differential voltages produced by each of the LVDT""s 100a and 100b with the instruction signals transmitted from CPU 5 to specify the lift of the flow control valve 1 and by monitoring the results of the comparison, a switch SWa is changed over in accordance with the results of monitoring.
The switch SWa is a switch for selecting which of the servo controllers 105a and 105b gives a control signal to a servo valve 4 that regulates the lift of the flow control valve 1 and it is switched over in accordance with the results of monitoring of each of the servo controllers 105a and 105b. Specifically, when the results of the comparison of the servo controllers 105a and 105b are different from each other and either of the servo controllers 105a and 105b is presumed to be at fault, switching over is controlled so that the servo valve 4 is given a control signal by the servo controller that is presumed not to be at fault.
However, when an LVDT is installed as shown in FIG. 5, for example, when the primary coil is broken, resulting in no flow of electric current through the coil, electric current is not induced in the secondary coil, and as a result, no current flows through the secondary coil. Thus, a differential voltage equivalent to that produced when the iron core is located in the center is given to the servo controller, causing the servo valve 4 to perform erroneously. In this way, depending on the state of the LVDT failure, a differential voltage within the normal range is sometimes given to the servo controller, which causes the flow control valve 1 to perform erroneously and move in a wrong direction.
Therefore, as in FIG. 5, even though feedback control is performed by two systems that use the LVDT""s 100a and 100b and even though the LVDT""s perform feedback control by utilizing one of the two systems that is operating normally, the operation actually performed may not always be normal but erroneous. Especially, in case of a flow control valve controlling the supply of fuels to a combustor which is used for a gas turbine, an erroneous operation makes an excessive amount of fuel flow into the combustor, which causes the temperature in the combustor to become dangerously high, thus finally leading to breakdown of the combustor.
In order to deal with the above-mentioned problem, an example of a control device is proposed in the Japanese Patent Application Laid-Open No. 2001-29118. In this publication, a fault is detected by checking the added values of the voltages generated in each of the secondary coils. However, the voltage of each of the secondary coils is not checked.
It is an object of the present invention to provide a flow control device which can detect a fault in a linear variable differential transformer that detects the lift of a flow control valve.
To achieve the above-mentioned object, in accordance with the present invention, a flow control device includes: a flow control valve controlling the flow of a fluid; first and second linear variable differential transformers that are equipped with a core moving in the same direction as the cylinder of the flow control valve and which detect the position of the valve cylinder; first and second servo controllers which generate control signals controlling the lift of the flow control valve based on signals detected individually by the first and the second linear variable differential transformers; and a switch which selects one of the control signals output individually from the first and the second servo controllers and supplies it to the flow control valve;
wherein the first and the second linear variable differential transformers each include: the core; a primary coil which is supplied with an alternating current voltage; two, first and second, secondary coils connected in parallel where voltages are included in accordance with the position of the core when said primary coil is energized with a voltage;
wherein the first servo controller monitors the voltages generated in the primary coil, the first secondary coil and the second secondary coil provided in the first linear variable differential transformer respectively; the second servo controller monitors the voltages generated in the primary coil, the first secondary coil and the second secondary coil provided in the second linear variable differential transformer respectively; while the control signal from the first servo controller is being selected by the switch, the switch stops selecting the control signal from the first servo controller on confirming that at least one of the primary coil, the first secondary coil and the second secondary coil provided in the first linear variable differential transformer outputs a voltage out of the range of normal performance; and while the control signal from the second servo controller is being selected by the switch, the switch stops selecting the control signal from the second servo controller on confirming that at least one of the primary coil, the first secondary coil and the second secondary coil provided in the second linear variable differential transformer outputs a voltage out of the range of normal performance.